Is it a No-Brainer to Grail Cu (Alter) over Mama Raikou?

I would say Raikou because Buster Crit meta and all that, but go with who you like and think you'll use more

Grailing on 5*s being inefficient aside I'd actually give the slight edge to Raikou in terms of gains from grails just because she needs the extra HP more while cu already has very strong defensive skills.

If we're ignoring stats and just going by preference though I'd give it to Cu over Raikou easily obviously this is very subjective though.

Don't do any of those two, each grail will only give them 2 levels more. For CuAlter it will bring his ATK from 12805 to 13043, and for Raikou it is from 13500 to 13753. Both get like 250 point increase and this does not translate directly to the damage they deal, so it is not really worth it. Spent it on your 3 star or 4 star servant, or even Spartacus to help farming.

u grail whoever you like that's it. Personally i like Cu alters playstyle way more because of his surviability and 2nd highest attack in the game. If i had him he'd be grailed to 100 in no time.